Grade 11

Grade 11Hydrogen


Hydrides and their classification


Hydrogen is the first and lightest element in the periodic table. It is a unique element, whose properties allow it to form compounds with almost every other element. These hydrogen compounds are known as hydrides. The study of hydrides is an essential part of chemistry, as these compounds are important both industrially and biologically.

The term "hydride" refers to a compound in which hydrogen is bonded to another element or group. Depending on the nature of the bond and the element combined with hydrogen, hydrides can be classified into different types. Understanding these classifications helps us predict the properties and reactivity of these compounds.

Classification of hydrides

Hydrides are broadly classified into three types:

  1. Molecular hydrides
  2. Ionic hydrides
  3. Metallic or interstitial hydrides

Molecular hydrides

Molecular hydrides, also called covalent hydrides, are formed when hydrogen forms covalent bonds with elements of similar or higher electronegativities. These are typically formed with non-metals, and the bond involves the sharing of electron pairs.

A common example of a molecular hydride is water (H 2 O). In water, each hydrogen atom shares one electron with an oxygen atom, forming a covalent bond. Below is a simplified electron-sharing diagram of water:

        H – O – H
    
Hey H H

Molecular hydrides can also be classified as electron-precise, electron-deficient, or electron-rich:

  • Electron-precise hydrides: These contain a sufficient number of bonds to satisfy the valency of the constituent atoms. For example, methane (CH 4) is electron-precise because it forms four covalent bonds with hydrogen.
  • Electron-deficient hydrides: These do not have enough electrons to form conventional covalent bonds. Borane (B 2 H 6) is an example of this, where the bonds are explained by three-center two-electron bonds, called banana bonds.
  • Electron rich hydrides: These contain extra pairs of electrons that do not participate in bonding, such as ammonia (NH 3).

Ionic hydrides

Ionic hydrides or saline hydrides are usually formed with metals, especially alkali metals and alkaline earth metals, which are highly electropositive. In these hydrides, the hydrogen atom gains an electron from the metal atom, forming a hydride ion H -.

Consider sodium hydride (NaH), an example of an ionic hydride. When sodium reacts with hydrogen, it transfers one electron to hydrogen, resulting in the sodium ion Na + and the hydride ion H -.

        Na + H → Na + + H -
    
Na + H -

Ionic hydrides are usually white crystalline solids. They are used in various chemical syntheses as a source of hydride ions and to transfer hydrogen in the form of hydride ions.

Metallic or interstitial hydrides

Metallic hydrides are formed when hydrogen interacts with transition metals. In these hydrides, the hydrogen atoms occupy interstitial sites within the metal lattice, i.e., the spaces between metal atoms. These are called interstitial hydrides.

Metallic hydrides often do not follow a simple stoichiometry, and their composition can vary. For example, titanium hydride and palladium hydride can absorb different amounts of hydrogen.

Metal atoms: grey squares, Hydrogen atoms: blue circles

Metallic hydrides, especially palladium hydrides, have fascinating properties, such as their ability to absorb and release hydrogen, making them useful for hydrogen storage solutions.

Applications of hydrides

Hydrides have various applications, which are influenced by their type and properties. Here are some examples:

  • Molecular Hydrides: Ammonia is used as a fertilizer and in the production of urea. Methane serves as an important energy source in natural gas.
  • Ionic hydrides: Sodium hydride is used as a drying agent and for deprotonation reactions in organic synthesis. Metal hydrides serve as reducing agents in metallurgical processes.
  • Metallic hydrides: These are important in hydrogen storage technologies, particularly as fuel cells and battery materials.

Reactivity of hydrides

The reactivity of hydrides varies widely depending on their classification:

  • Molecular hydrides: These can be relatively inert like methane or highly reactive like diborane. The presence of lone pairs in compounds like ammonia contributes to their basic and nucleophilic properties.
  • Ionic hydrides: These react vigorously with water, releasing hydrogen gas. For example, the reaction of sodium hydride with water:
                    NaH + H 2 O → NaOH + H 2
  • Metallic hydrides: They often exhibit low reactivity due to the stability provided by the metal lattice. However, under the right conditions, they can release hydrogen gas.

Conclusion

Hydrides are a fascinating group of compounds with diverse structures and properties. Their classification into molecular, ionic and metallic hydrides helps us understand their unique behaviour and applications in various scientific and industrial fields. From energy storage to synthetic chemistry, hydrides provide important applications that are indispensable to modern science and technology.

As chemistry continues to develop, research on hydrides will expand and deepen, revealing more insights into sustainable energy solutions and advanced materials, making hydrides an exciting area of study for future chemists.


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